High speed actuators are desirable in information storage disk drive systems to reduce access times during track seek operations thereby providing improved system data throughput performance. As is known, the speed of the actuator is directly related to the force that the motor can apply to the moving mass of the disk read mechanism since greater forces produce greater accelerations and thus reduce the time required to move the read mechanism from one data track on the disk to another, an operation commonly referred to as a "seek".
The use of linear ("voice coil") motors as disk drive actuators is well known. In this type of motor, the force generated is proportional to the number of turns in the electromagnetic drive coil, the amount of current flowing in the wire of the coil and the strength of the magnetic field through which the wire passes. As the actuator moves through its stroke, the coil and its current may be thought of as being constant while the magnetic field generally tends to diminish ("roll off") towards the extreme positions of the field. This is due to flux leakage occurring at the ends of the magnetic circuit where the geometry of the circuit inherent provides additional leakage path. Servo control circuits used to drive actuators usually must operate with reduced overall performance in order to accommodate this reduction in available acceleration/deceleration capability at the ends of the stroke. It is desirable therefore that forces in voice coil actuators be made uniform or constant to improve the operating performance of the actuator.
One known approach to making the forces uniform in this type of actuator is to lengthen the magnetic circuit is order to move the ends, and therefore the end effects, outward well past the travel of the actuator coil where these end effects then have less effect on usable stroke. This approach has the disadvantage of increasing the total flux generated within the magnetic circuit therefore increasing the total flux that must be carried by the center pole of the circuit. This additional flux requires that a center pole with a larger cross section be used. This in turn would mean that the diameter of the coil would have to be increased to encompass the larger center pole. Another approach, to compensate for larger magnetic circuits, is to decrease the average flux density of the field of the circuit. The end result of these approaches is either an increase in size of the coil with consequent unwanted increase in moving mass or an overall decrease in the forces produced by the motor.
A second approach has been to vary the size of the magnetic gap in which the coil travels along its stroke. A narrower gap at the ends of the stroke increases the flux density in these regions thus reducing the end effects. However, this approach has the disadvantage of aggravating clearance and tolerance problems and is generally not considered practical to implement in volume manufacturing operations.
It is therefore an object of the invention to provide an improved voice coil motor with a uniform force constant which avoids the problems and disadvantages of the prior art.
It is another object of the invention to provide an improved voice coil motor of the type described that maximizes the efficiency of a size-constrained design.
It is yet another object of the invention to provide a tracking actuator for an optical beam addressable information storage disk drive system utilizing an improved voice coil motor of the type described.